This invention relates to the manufacture of hearing aids. In general, hearing aids receive sound signals and amplify, or otherwise modify, the signals for the hearing aid user. The present invention particularly relates to hearing aids that attempt to amplify speech signals more than noise signals and thus improve the clarity of the signal ultimately provided by the hearing aid to the user.
Electrical circuits for hearing aids circuits are well known. An example of one such hearing aid circuit is described in U.S. Pat. No. 4,792,977 by James Anderson and Richard Brander.
Since hearing aids are worn on or in the human ear, the hearing aid circuitry and battery must necessarily be physically small. Consequently, such circuitry must necessarily be compact and use low levels of power. Nonetheless, the circuitry should also be as complex as necessary to provide a proper signal to help the hearing aid user with his or her particular hearing deficiency.
Many hearing aid users, for example, have difficulty in understanding speech when background noises are simultaneously present. The circuitry for hearing aids used by such persons should attempt to amplify speech signals more than the noise signal. The ability of the hearing aid circuitry to thus "discriminate" between speech and noise, and adjust amplification levels accordingly, significantly improves the effectiveness of the hearing aid and, thus, the hearing ability of the hearing aid user.
Unfortunately, many of the available noise discrimination techniques are poorly suited for use within a hearing aid. Many circuits useful to implement the techniques are physically large or consume significant amounts of electrical power. Other such circuits are simply expensive to manufacture or provide minimal discrimination between speech and noise signals, or have undesirable side effects on the reproduced sound. Prior art devices have used the fact that most noise signals contain high levels of low frequency energy and use the presence of this energy to reduce low frequency amplification if a threshold level is exceeded. Unfortunately, speech also contains significant low frequency energy which may trigger such circuits to reduce low frequency gain even when background noise is not present, reducing naturalness of reproduction.
It would be desirable to take into account the time characteristics of the envelope of the signal to distinguish between steady state noise-like signals and dynamically varying signals, such as speech. This is a feature of the present invention.
Other examples of prior art devices are disclosed in U.S. Pat. Nos. 4,490,585 and 4,409,435.